Methods and apparatuses for filtering common mode interference

ABSTRACT

In one embodiment, a method that includes detecting a level on each of a plurality of data channels; summing the levels of a subset of data channels to get a summed level, the subset of data channels containing the lowest levels present in the plurality of data channels; comparing the summed level with the highest level present in the plurality of data channels; and rejecting a common mode interference in each of the plurality of data channels if the summed level exceeds the highest level present in the plurality of data channels. In another embodiment, an apparatus that includes a plurality of data channels; an array of input filters coupled to the plurality of data channels; a pulse detection circuit coupled to the array of input filters; a common mode rejection circuit coupled to the pulse detection circuit; and a pulse data queuing and transmission circuit coupled to the common mode rejection circuit. Other methods and apparatuses are disclosed.

CROSS-REFERENCE(S) TO RELATED APPLICATION(S)

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 60/437,171, filed Dec. 30, 2002, the entire contents of whichare expressly incorporated by reference.

REFERENCE TO APPENDIX

This application includes a computer program listing appendix, submittedon compact disc (CD). The content of the CD is incorporated by referencein its entirety and accordingly forms a part of this specification. TheCD contains the following file:

File name: full7function.txt

File Size: 35.1 kb

Creation date for CD: Jun. 25, 2003

BACKGROUND OF THE INVENTION

The portion of this disclosure contained on CD of this patent documentcontains material that is subject to copyright protection. The copyrightowner has no objection to the facsimile reproduction by anyone of thepatent document or the patent disclosure on the CD, as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright rights whatsoever.

1. Field of the Invention

The invention relates generally to the field of signal processing. Moreparticularly, the invention relates to a method and apparatus forfiltering common mode interference.

2. Discussion of the Related Art

Typically, multi-channel navigation systems include an array of sensors(or transducers) and signal conditioning elements for detecting analogsignals and generating digital data for further processing by a guidanceprocessor. The guidance processor can determine the position of a signalsource based on the received information while using parameterscharacteristic of the array. A multi-channel navigation system may be,for example, a multi-channel laser seeker.

A multi-channel laser seeker receives laser pulses on a few of itsmultiple channels, and uses this data to determine the direction orposition of a laser source. In practice, the seeker receives data onlyon a small subset of its channels for any given laser pulse, while theother channels are near zero on their response.

Unfortunately, multi-channel laser seekers may be subject to electricalinterference that may appear as large signals on most or all channelssimultaneously. The electrical interference may be in the form of pulsesand, when the average pulse repetition frequencies (PRF) is high, it mayseverely degrade the seeker's performance.

SUMMARY OF THE INVENTION

In one embodiment, the invention is a method that includes detecting alevel on each of a plurality of data channels; summing the levels of asubset of data channels to get a summed level, the subset of datachannels containing the lowest levels present in the plurality of datachannels; comparing the summed level with the highest level present inthe plurality of data channels; and rejecting a common mode interferencein each of the plurality of data channels if the summed level exceedsthe highest level present in the plurality of data channels. Otherembodiments of the present methods are described below.

In another embodiment, the invention is an apparatus that includes aplurality of data channels; an array of input filters coupled to theplurality of data channels; a pulse detection circuit coupled to thearray of input filters; a common mode rejection circuit coupled to thepulse detection circuit, the common mode rejection circuit operable tosum the levels of a subset of data channels to get a summed level, thesubset of data channels containing the lowest levels present in theplurality of data channels, compare the summed level with the highestlevel present in the plurality of data channels, and reject a commonmode interference in each of the plurality of data channels if thesummed level exceeds the highest level present in the plurality of datachannels; and a pulse data queuing and transmission circuit coupled tothe common mode rejection circuit. Other embodiments of the presentapparatuses are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings demonstrate aspects of the present methods andapparatuses. They illustrate by way of example and not limitation. Likereference numbers refer to similar elements. The features illustrated inthe drawings are not necessarily drawn to scale.

FIGS. 1A-B show a block diagram of one embodiment of a common modeinterference processing system suitable for use with the present methodand apparatuses.

FIG. 2 shows a flowchart of one embodiment of a common mode interferencefiltering method suitable for use with the present method andapparatuses.

FIG. 3 shows a graph of filtered and unfiltered simulated data streams,illustrating an aspect of the invention.

DETAILED DESCRIPTION

In this document (including the claims), the terms “comprise” (and anyform of comprise, such as “comprises” and “comprising”), “have” (and anyform of have, such as “has” and “having”), and “include” (and any formof include, such as “includes” and “including”) are open-ended linkingverbs. For example, a method “comprising” detecting a level on each of aplurality of data channels; summing the levels of a subset of datachannels to get a summed level; comparing the summed level with thehighest level present in the plurality of data channels; and rejecting acommon mode interference in each of the plurality of data channels ifthe summed level exceeds the highest level present in the plurality ofdata channels is a method that possesses at least these steps, but isnot limited to possessing only these steps.

Similarly, an apparatus “comprising” a plurality of data channels; anarray of input filters coupled to the plurality of data channels; apulse detection circuit coupled to the array of input filters; a commonmode rejection circuit coupled to the pulse detection circuit; and apulse data queuing and transmission circuit coupled to the common moderejection circuit is an apparatus that possesses at least thesestructures, but is not limited to possessing only these structures. Forexample, this apparatus also covers an apparatus that possesses aguidance circuit coupled to the pulse data queuing and transmissioncircuit.

The terms “a” and “an” are defined as one or more than one. The term“another” is defined as at least a second or more. The term “plurality”is defined as two or more than two. The term “coupled” is defined asconnected, although not necessarily directly, and not necessarilymechanically. The term “program” or “computer program” is defined as asequence of instructions designed for execution on a computer system.

Those of ordinary skill in the art will appreciate that in the detaileddescription below, certain well known components and assembly techniqueshave been omitted so that the invention is not obscured in unnecessarydetail.

The invention may include a method and apparatus for filtering a commonmode interference embedded in a plurality of data streams. In oneexemplary embodiment, the invention may include a digital front-end thatremoves a common mode interference from the plurality of data streamsbefore the data is sent to a pulse processor or a guidance circuit. Inanother embodiment, the invention may include rejecting a common modeinterference in a multi-channel laser seeker to determine the directionor position of a laser source. The laser seeker may have, for example,seven data channels. Such systems may find a variety of applicationsincluding, for example, navigation and automated projectile maneuvering.

Typically, a multi-channel laser seeker receives data only on a smallsubset of its channels for any given laser pulse, while the otherchannels are near zero on their response. Nevertheless, the seeker maybe subject to electrical interference on most or all channelssimultaneously. The invention may include identifying and rejectingcommon mode interference in a plurality of channels with a digitalfront-end.

Referring to FIGS. 1A-B, a block diagram of one embodiment of a commonmode interference processing system 100 is depicted. Plurality ofdigitized data channels 101 is coupled to array of input filters 105 ofdigital front-end circuit 130. Array of input filters 105 is coupled topulse detector circuit 110, and pulse detector circuit 110 is coupled tocommon mode rejection circuit 115. Common mode rejection circuit 115 iscoupled to pulse data queuing and transmission circuit 120, and pulsedata queuing and transmission circuit 120 is coupled to pulse processor140 of guidance circuit 135. Pulse processor 140 is coupled to commonmode rejection filter 115, angle determination circuit 145, andautomatic gain control and threshold circuit 150. Automatic gain controland threshold circuit 150 is coupled to a variable gain amplifier (notshown) and to pulse detection circuit 110. Angle determination circuit145 is coupled to a computer (not shown). Storage media 155 is coupledto angle determination circuit 145.

In one exemplary embodiment, each of plurality of digitized datachannels 101 may represent an incoming digitized signal from a sensor ora transducer, such as, for example, a photodiode or the like.

In practice, digital front-end circuit 130 may be an integrated circuitor a programmable circuit, such as, for example, a programmable logicdevice (PLD) or a field-programmable gate array (FPGA). When digitalfront-end circuit 130 is a programmable circuit, a program, such as thatpresented below and discussed in detail with reference to FIG. 2,creates an apparatus in accordance with the present invention thatoperates in accordance with the methods of the present invention. In thealternative, digital front-end circuit 130 may be hard-wired or may usepredetermined data tables, or may be a combination of hard-wired andprogrammable circuitry.

When in operation, a plurality of incoming digitized signals entersdigital front-end circuit 130 via array of input filters 106. An inputfilter may be, for example, high-pass filters to correct for ananalog-to-digital conversion effect. Next, the plurality of signals isfed into pulse detection circuit 110, which identifies pulses accordingto a threshold set by automatic gain control and threshold circuit 150,delivering a plurality of unfiltered data streams to common moderejection circuit 108. At common mode rejection circuit 108, a commonmode interference is filtered out of the data streams, and the datastreams are transmitted to guidance circuit 135 via pulse data queuingand transmission circuit 120.

Guidance circuit 135 may be a processor, a digital signal processor(DSP), a computer, or the like. Program storage media 155 may be anytype of readable memory including, for example, a magnetic or opticalmedia such as a card, tape or disk, or a semiconductor memory such as aPROM or FLASH memory. In one embodiment, program storage media 155 mayinclude a stored seeker response data.

When in operation, guidance circuit 135 processes streams of pulses anddetermines the direction of a source by calculating angles based onpulse amplitudes and using a calibration data such as, for example, aseeker's response data stored in storage media 155. The operation ofguidance circuit 135 is known to one of ordinary skill in the art andneeds not be detailed here.

Referring to FIG. 2, a flowchart of one embodiment of a common modeinterference filtering method 200 is depicted. Method 200 may beperformed by common mode rejection circuit 115 of FIG. 1A. In oneembodiment, the method monitors a plurality of data channels for pulses,performs a calculation with a number of channels that have the lowestvoltage level, and compares this result with the channel that has thehighest level.

In step 205, a pulse is received. In step 210, a subset of N channelswith the lowest levels among the plurality of data channels isidentified, and these lowest levels are summed into a summed level,where N is an integer. In one embodiment, N is equal to 3—that is, 3data channels with the 3 lowest levels are summed. In step 215, asensitivity factor provided by the pulse processor 140 to the commonmode rejection circuit 115 is used to multiply the sum of N channels. Inone embodiment, the sensitivity factor is set to 1. In anotherembodiment, the sensitivity factor is equal to 2. In step 220, thesummed level is compared to the highest level present among theplurality of data channels. If the sum of lowest N channels if largerthan the highest channel level, the pulses on every channel are deleted.Otherwise the channels data is transmitted to guidance circuit 140detailed in FIG. 1B. Next, control passes to step 235 and the methodwaits for the next pulse.

In one embodiment, method 200 is repeated several times. In anotherembodiment, method 200 is repeated at every data point. The inventionmay include performing method 200 every 10 ηs (100 MHz). The sensitivityfactor may be changed by pulse processor 140 as a function of the falsealarm rate, that is, the rate at which method 200 deletes actual datainstead of filtering only common mode interference.

Software or computer instructions configured to carry out one or moresteps of method 200 may be loaded onto a computer readable medium. Oneof ordinary skill in the art will understand that computer readablemedium may take many forms, including any data storage device that canstore data that can thereafter be read by a processor, a computer or acomputer system, including, for example, a disk, such as a floppy disk,a zip disk, or the like; read-only memory; random access memory;CD-ROMs; magnetic tape; optical data storage devices, SMARTMEDIA® cards;flash memory; compact flash memory; and the like. The computer readablemedium can also be distributed over network-coupled computer systems sothat the computer readable instructions are stored and executed in adistributed fashion. For example, the computer readable medium may alsotake the form of a carrier wave such as, for example, signals on a wire(e.g., signals downloaded from the Internet) or those that aretransmitted electromagnetically or through infra red means.

Shown in the computer program listing appendix (see CD) is an exemplarysource code written in Altera Hardware Description Language that issuitable for carrying out steps described above, and which may be usedin conjunction with a field programmable gate array (FPGA) such as, forexample, the APEX 20KE FPGA (EP20K200E) from Altera Corporation. Thecode is an example of how to filter a common mode interference accordingto the methods detailed in FIG. 2 and described above. This code isexemplary and does not limit the scope of the claims. It simplyrepresents one specific embodiment for carrying out steps associatedwith the present methods and is included for the convenience of thereader in this regard. Those of ordinary skill in the art having thebenefit of this disclosure will recognize that a wide variety ofcomputational techniques and/or different types of corresponding sourcecode may be used in implementing the present methods.

Referring to FIG. 3, a graph of filtered and unfiltered simulated datastreams 400 is depicted illustrating an aspect of the invention. Thehorizontal axis is time and the vertical axis is the amplitude.Plurality of incoming channels 301A-304A containing common modeinterference is processed by common mode rejection circuit 115 of FIG.1A operating as described above. Resulting filtered data streams301B-304B contain only data pulses 310 in channel 301B, and are free ofthe common mode interference originally present in every incomingchannel 301A-304A.

The individual components described above need not be made in the exactdisclosed forms, or combined in the exact disclosed configurations, butcould be provided in any suitable form, and/or combined in any suitableconfiguration. It will also be clear to those of ordinary skill in theart substitutions, modifications, additions and/or rearrangements of thefeatures of the inventive methods and devices may be made withoutdeviating from their scope, which is defined by the claims and theirequivalents. The appended claims are not to be interpreted as includingmeans-plus-function limitations, unless such a limitation is explicitlyrecited in a given claim using the phrase(s) “means for” and/or “stepfor,” respectively.

1. A method comprising: detecting a level on each of a plurality of datachannels; summing the levels of a subset of data channels to get asummed level, the subset of data channels containing the lowest levelspresent in the plurality of data channels; comparing the summed levelwith the highest level present in the plurality of data channels; andrejecting a common mode interference in each of the plurality of datachannels if the summed level exceeds the highest level present in theplurality of data channels.
 2. The method of claim 1, where thedetecting includes monitoring every data point of the plurality of datachannels.
 3. The method of claim 1, where the detecting includesdetecting a level on each of seven data channels.
 4. The method of claim1, where the detecting includes detecting a pulse on each of a pluralityof data channels.
 5. The method of claim 1, where the summing the levelsof the subset of data channels includes summing the levels of three datachannels.
 6. The method of claim 5, where the summing the levels ofthree data channels includes summing the three lowest levels present inthe plurality of data channels.
 7. The method of claim 1, furthercomprising multiplying the summed level by a sensitivity factor.
 8. Themethod of claim 7, further comprising adjusting the sensitivity factorto minimize a false alarm rate.
 9. An apparatus comprising: a pluralityof data channels; an array of input filters coupled to the plurality ofdata channels; a pulse detection circuit coupled to the array of inputfilters; a common mode rejection circuit coupled to the pulse detectioncircuit, the common mode rejection circuit operable to: sum the levelsof a subset of data channels to get a summed level, the subset of datachannels containing the lowest levels present in the plurality of datachannels, compare the summed level with the highest level present in theplurality of data channels, and reject a common mode interference ineach of the plurality of data channels if the summed level exceeds thehighest level present in the plurality of data channels; and a pulsedata queuing and transmission circuit coupled to the common moderejection circuit.
 10. The apparatus of claim 9, further comprising aguidance circuit coupled to the pulse data queuing and transmissioncircuit.